As each kingdom have a different color, frontiers are easily visibles, you can add plain or dashed lines if you want but this is not mandatory. For the coloring, you could try to lower the opacity of the background and add a blurred inner border of the same color, a bit like this one : https://www.cartographersguild.com/a...chmentid=25591 Just an example, it is the first I piked on the finished maps gallery. The frontiers are maybe a bit straight, good locations are mountains and sometimes rivers. I find also that the outer lines around the landmasses are too mechanical, you could maybe reduce the number of lines and/or distord the selection a bit before stroking the line on it ?
Good luck and carry on!